Field of the Invention
The present invention relates to a sensor control method and a sensor control apparatus for controlling a sensor suitable for detection of the concentrations of nitrogen oxides and ammonia contained in a gas under measurement.
Description of Related Art
In recent years, a urea SCR (Selective Catalytic Reduction) system has drawn people's attention as a technique for purifying nitrogen oxides (NOx) contained in exhaust gas discharged from an internal combustion engine such as a gasoline engine, a diesel engine, or the like. The urea SCR system is a system which chemically reacts ammonia (NH3) with nitrogen oxides (NOx) so as to reduce the nitrogen oxides to nitrogen (N2), to thereby purify the nitrogen oxides contained in the exhaust gas.
This urea SCR system has a possibility that when the amount of ammonia supplied to nitrogen oxides becomes excessively large, unreacted ammonia is discharged to the outside in a state in which it is contained in the exhaust gas. In order to suppress discharge of such ammonia, a sensor which can measure the concentrations of a plurality of types of gases and which includes a sensor element for measuring the concentration of ammonia contained in exhaust gas is used for the urea SCR system.
In the urea SCR system, the amount of ammonia used for reduction of nitrogen oxides is adjusted such that the concentration of ammonia measured by the above-described sensor; i.e., the concentration of ammonia contained in exhaust gas, falls within a predetermined range.
Notably, an example of such a sensor is that which has an NOx sensor section and an ammonia sensor section. Also, examples of the configuration of such a sensor include a configuration in which an NOx sensor section and an ammonia sensor section are provided individually and a configuration in which an NOx sensor section and an ammonia sensor section are integrated together (multi-gas sensor). Nitrogen monoxide (NO), nitrogen dioxide (NO2) and ammonia (NH3) can be detected through use of such a sensor.
Also, detection of ammonia concentration by the ammonia sensor section involves a possibility that the ammonia concentration varies due to the influence of oxygen concentration and the accuracy in detecting the ammonia concentration decreases. Therefore, there has been proposed a technique for calculating corrected ammonia concentration on the basis of an output signal of the ammonia sensor section and oxygen concentration (for example, see Patent Document 1). Such a technique for calculating corrected ammonia concentration can reduce the influence of the oxygen concentration in a gas under measurement, to thereby suppress a decrease in the accuracy in detecting the ammonia concentration.